;;; Common Lisp backquote implementation, written in Common Lisp.
;;; Author: Guy L. Steele Jr.     Date: 27 December 1985
;;; Tested under Symbolics Common Lisp and Lucid Common Lisp.
;;; This software is in the public domain.

(in-package :sys.int)

;;; The following are unique tokens used during processing.
;;; They need not be symbols; they need not even be atoms.

(eval-when (:compile-toplevel :load-toplevel :execute)

(defvar *comma* 'bq-comma)
(defvar *comma-atsign* 'bq-comma-atsign)
(defvar *comma-dot* 'bq-comma-dot)
(defvar *bq-list* (make-symbol "BQ-LIST"))
(defvar *bq-append* (make-symbol "BQ-APPEND"))
(defvar *bq-list** (make-symbol "BQ-LIST*"))
(defvar *bq-nconc* (make-symbol "BQ-NCONC"))
(defvar *bq-clobberable* (make-symbol "BQ-CLOBBERABLE"))
(defvar *bq-quote* (make-symbol "BQ-QUOTE"))
(defvar *bq-quote-nil* (list *bq-quote* nil))

;;; Reader macro characters:
;;;    $foo is read in as (BACKQUOTE foo)
;;;    %foo is read in as (#:COMMA foo)
;;;    %@foo is read in as (#:COMMA-ATSIGN foo)
;;;    %.foo is read in as (#:COMMA-DOT foo)

;;; where #:COMMA is the value of the variable *COMMA*, etc.
;;; BACKQUOTE is an ordinary macro (not a read-macro) that
;;; processes the expression foo, looking for occurrences of
;;; #:COMMA, #:COMMA-ATSIGN, and #:COMMA-DOT.  It constructs code
;;; in strict accordance with the rules on pages 349-350 of
;;; the first edition (pages 528-529 of this second edition).
;;; It then optionally applies a code simplifier.

;; (set-macro-character #\$
;;		     #'(lambda (stream char)
;;			 (declare (ignore char))
;;			 (list 'backquote (read stream t nil t))))

;;(set-macro-character #\%
;;		     #'(lambda (stream char)
;;			 (declare (ignore char))
;;			 (case (peek-char nil stream t nil t)
;;			   (#\@ (read-char stream t nil t)
;;				(list *comma-atsign* (read stream t nil t)))
;;			   (#\. (read-char stream t nil t)
;;				(list *comma-dot* (read stream t nil t)))
;;			   (otherwise (list *comma* (read stream t nil t))))))

;;; If the value of *BQ-SIMPLIFY* is non-NIL, then BACKQUOTE
;;; processing applies the code simplifier.  If the value is NIL,
;;; then the code resulting from BACKQUOTE is exactly that
;;; specified by the official rules.

(defparameter *bq-simplify* t)

(defmacro backquote (x)
  (bq-completely-process x))

;;; Backquote processing proceeds in three stages:
;;;
;;; (1) BQ-PROCESS applies the rules to remove occurrences of
;;; #:COMMA, #:COMMA-ATSIGN, and #:COMMA-DOT corresponding to
;;; this level of BACKQUOTE.  (It also causes embedded calls to
;;; BACKQUOTE to be expanded so that nesting is properly handled.)
;;; Code is produced that is expressed in terms of functions
;;; #:BQ-LIST, #:BQ-APPEND, and #:BQ-CLOBBERABLE.  This is done
;;; so that the simplifier will simplify only list construction
;;; functions actually generated by BACKQUOTE and will not involve
;;; any user code in the simplification.  #:BQ-LIST means LIST,
;;; #:BQ-APPEND means APPEND, and #:BQ-CLOBBERABLE means IDENTITY
;;; but indicates places where "%." was used and where NCONC may
;;; therefore be introduced by the simplifier for efficiency.
;;;
;;; (2) BQ-SIMPLIFY, if used, rewrites the code produced by
;;; BQ-PROCESS to produce equivalent but faster code.  The
;;; additional functions #:BQ-LIST* and #:BQ-NCONC may be
;;; introduced into the code.
;;;
;;; (3) BQ-REMOVE-TOKENS goes through the code and replaces
;;; #:BQ-LIST with LIST, #:BQ-APPEND with APPEND, and so on.
;;; #:BQ-CLOBBERABLE is simply eliminated (a call to it being
;;; replaced by its argument).  #:BQ-LIST* is replaced by either
;;; LIST* or CONS (the latter is used in the two-argument case,
;;; purely to make the resulting code a tad more readable).

(defun bq-completely-process (x)
  (let ((raw-result (bq-process x)))
    (bq-remove-tokens (if *bq-simplify*
                          (bq-simplify raw-result)
                          raw-result))))

(defun bq-process (x)
  (cond ((atom x)
         (list *bq-quote* x))
        ((eq (car x) 'backquote)
         (bq-process (bq-completely-process (cadr x))))
        ((eq (car x) *comma*) (cadr x))
        ((eq (car x) *comma-atsign*)
         (error ",@~S after `" (cadr x)))
        ((eq (car x) *comma-dot*)
         (error ",.~S after `" (cadr x)))
        (t (do ((p x (cdr p))
                (q '() (cons (bracket (car p)) q)))
               ((atom p)
                (cons *bq-append*
                      (nreconc q (list (list *bq-quote* p)))))
             (when (eq (car p) *comma*)
               (unless (null (cddr p)) (error "Malformed ,~S" p))
               (return (cons *bq-append*
                             (nreconc q (list (cadr p))))))
             (when (eq (car p) *comma-atsign*)
               (error "Dotted ,@~S" p))
             (when (eq (car p) *comma-dot*)
               (error "Dotted ,.~S" p))))))

;;; This implements the bracket operator of the formal rules.

(defun bracket (x)
  (cond ((atom x)
         (list *bq-list* (bq-process x)))
        ((eq (car x) *comma*)
         (list *bq-list* (cadr x)))
        ((eq (car x) *comma-atsign*)
         (cadr x))
        ((eq (car x) *comma-dot*)
         (list *bq-clobberable* (cadr x)))
        (t (list *bq-list* (bq-process x)))))

;;; This auxiliary function is like MAPCAR but has two extra
;;; purposes: (1) it handles dotted lists; (2) it tries to make
;;; the result share with the argument x as much as possible.

(defun maptree (fn x)
  (if (atom x)
      (funcall fn x)
      (let ((a (funcall fn (car x)))
            (d (maptree fn (cdr x))))
        (if (and (eql a (car x)) (eql d (cdr x)))
            x
            (cons a d)))))

;;; This predicate is true of a form that when read looked
;;; like %@foo or %.foo.

(defun bq-splicing-frob (x)
  (and (consp x)
       (or (eq (car x) *comma-atsign*)
           (eq (car x) *comma-dot*))))

;;; This predicate is true of a form that when read
;;; looked like %@foo or %.foo or just plain %foo.

(defun bq-frob (x)
  (and (consp x)
       (or (eq (car x) *comma*)
           (eq (car x) *comma-atsign*)
           (eq (car x) *comma-dot*))))

;;; The simplifier essentially looks for calls to #:BQ-APPEND and
;;; tries to simplify them.  The arguments to #:BQ-APPEND are
;;; processed from right to left, building up a replacement form.
;;; At each step a number of special cases are handled that,
;;; loosely speaking, look like this:
;;;
;;;  (APPEND (LIST a b c) foo) => (LIST* a b c foo)
;;;       provided a, b, c are not splicing frobs
;;;  (APPEND (LIST* a b c) foo) => (LIST* a b (APPEND c foo))
;;;       provided a, b, c are not splicing frobs
;;;  (APPEND (QUOTE (x)) foo) => (LIST* (QUOTE x) foo)
;;;  (APPEND (CLOBBERABLE x) foo) => (NCONC x foo)

(defun bq-simplify (x)
  (if (atom x)
      x
      (let ((x (if (eq (car x) *bq-quote*)
                   x
                   (maptree #'bq-simplify x))))
        (if (not (eq (car x) *bq-append*))
            x
            (bq-simplify-args x)))))

(defun bq-simplify-args (x)
  (do ((args (reverse (cdr x)) (cdr args))
       (result
	nil
	(cond ((atom (car args))
	       (bq-attach-append *bq-append* (car args) result))
	      ((and (eq (caar args) *bq-list*)
		    (notany #'bq-splicing-frob (cdar args)))
	       (bq-attach-conses (cdar args) result))
	      ((and (eq (caar args) *bq-list**)
		    (notany #'bq-splicing-frob (cdar args)))
	       (bq-attach-conses
		(reverse (cdr (reverse (cdar args))))
		(bq-attach-append *bq-append*
				  (car (last (car args)))
				  result)))
	      ((and (eq (caar args) *bq-quote*)
		    (consp (cadar args))
		    (not (bq-frob (cadar args)))
		    (null (cddar args)))
	       (bq-attach-conses (list (list *bq-quote*
					     (caadar args)))
				 result))
	      ((eq (caar args) *bq-clobberable*)
	       (bq-attach-append *bq-nconc* (cadar args) result))
	      (t (bq-attach-append *bq-append*
				   (car args)
				   result)))))
      ((null args) result)))

(defun null-or-quoted (x)
  (or (null x) (and (consp x) (eq (car x) *bq-quote*))))

;;; When BQ-ATTACH-APPEND is called, the OP should be #:BQ-APPEND
;;; or #:BQ-NCONC.  This produces a form (op item result) but
;;; some simplifications are done on the fly:
;;;
;;;  (op '(a b c) '(d e f g)) => '(a b c d e f g)
;;;  (op item 'nil) => item, provided item is not a splicable frob
;;;  (op item 'nil) => (op item), if item is a splicable frob
;;;  (op item (op a b c)) => (op item a b c)

(defun bq-attach-append (op item result)
  (cond ((and (null-or-quoted item) (null result))
         (list *bq-quote* (cadr item)))
        ((and (null-or-quoted item) (null-or-quoted result))
         (list *bq-quote* (append (cadr item) (cadr result))))
        ((or (null result) (equal result *bq-quote-nil*))
         (if (bq-splicing-frob item) (list op item) item))
        ((and (consp result) (eq (car result) op))
         (list* (car result) item (cdr result)))
        (t (list op item result))))

;;; The effect of BQ-ATTACH-CONSES is to produce a form as if by
;;; `(LIST* ,@items ,result) but some simplifications are done
;;; on the fly.
;;;
;;;  (LIST* 'a 'b 'c 'd) => '(a b c . d)
;;;  (LIST* a b c 'nil) => (LIST a b c)
;;;  (LIST* a b c (LIST* d e f g)) => (LIST* a b c d e f g)
;;;  (LIST* a b c (LIST d e f g)) => (LIST a b c d e f g)

(defun bq-attach-conses (items result)
  (cond ((and (every #'null-or-quoted items)
              (null-or-quoted result))
         (list *bq-quote*
               (append (mapcar #'cadr items) (cadr result))))
        ((or (null result) (equal result *bq-quote-nil*))
         (cons *bq-list* items))
        ((and (consp result)
              (or (eq (car result) *bq-list*)
                  (eq (car result) *bq-list**)))
         (cons (car result) (append items (cdr result))))
        (t (cons *bq-list** (append items (list result))))))

;;; Removes funny tokens and changes (#:BQ-LIST* a b) into
;;; (CONS a b) instead of (LIST* a b), purely for readability.

(defun bq-remove-tokens (x)
  (cond ((eq x *bq-list*) 'list)
        ((eq x *bq-append*) 'append)
        ((eq x *bq-nconc*) 'nconc)
        ((eq x *bq-list**) 'list*)
        ((eq x *bq-quote*) 'quote)
        ((atom x) x)
        ((eq (car x) *bq-clobberable*)
         (bq-remove-tokens (cadr x)))
        ((and (eq (car x) *bq-list**)
              (consp (cddr x))
              (null (cdddr x)))
         (cons 'cons (maptree #'bq-remove-tokens (cdr x))))
        (t (maptree #'bq-remove-tokens x))))

)
